I had a bike that did that...problem was getting too much fuel! In top gear you are pullling the throttle more but the engine is not doing enough revs to burn it off. Is it worse if you are going up a hill and better if you are going down a hill? Mine was fine until you put in into 5th and tried to keep accelerating but if you dropped it into 4th and kept the same speed it was fine. Found out he float level was wrong in the carb.Sorted it out and it was fine then.
Steve
Steve
